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ScienceWise - May/Jun 2006

Reading the Sun in the Moon

Article Illustration
Dr Trevor Ireland, seen here holding a meteorite, has long been fascinated by the chemical composition of the early solar system. Using the SHRIMP to decipher the isotopic composition and age of meteorites, lunar soil and solar wind, it’s possible to build a model of how the Solar System and our planet formed.
Article Illustration
A scanning electron micrograph of one of the iron metal grains analysed from the lunar soil sample.
Article Illustration
An artist’s conception of the Genesis probe with solar wind collectors deployed. (Courtesy of JPL/LMA)

What’s the chemical make up of the Sun and the proto-planetary soup that gave birth to our solar system? These are burning questions in astrophysics and the earth sciences, and part of the answer is believed to lie in samples of soil collected from the Moon by Neil Armstrong back in 1969.

Why would you look to the Moon when you’re trying to understand the composition of the Sun?

“We can’t get samples directly from the Sun,” explains Dr Trevor Ireland from the Research School of Earth Sciences. “However we can infer its composition by looking at lunar samples, which are believed to reflect its composition. This is because lunar soil contains oxygen isotopes ‘implanted’ by solar winds carrying elements blown out from the Sun.”

To date, the isotopic composition of the Sun has been inferred for some elements from lunar samples, but not for oxygen because of its high abundance in lunar minerals. However, a new analysis using the Sensitive High-Resolution Ion Microprobe (also known as the SHRIMP) has been able to get an accurate measurement of the solar oxygen isotopes in iron metal grains. These metal grains have very low intrinsic oxygen and so the solar wind implant dominates the signal from these grains.

In particular, researchers hoped to find evidence for either of the two reigning theories about the Sun’s composition. According to one theory, the Sun has a similar oxygen composition to the planets and meteorities. The other theory suggests it has enriched levels of the isotope

oxygen-16.

However, rather than producing a result that supported one of the theories, their analysis came up with something neither of them predicted – it has lower levels of oxygen-16 than expected (and the oxygen signal is dissimilar to bodies like the Earth and meteorites).

“We found that the oxygen isotope levels did not agree with either a planetary composition or the oxygen-16 rich composition,” says Dr Ireland.

“The oxygen isotopes are telling us that the mix of components in the Sun is different to that in the planets, particularly in regard to the amount of dust versus gas that comprises the Sun versus the planets.”

“This was a completely unexpected result for us,” says Dr Ireland. “Our Sun is not the Sun that we thought it was.

“The finding also suggests that the Sun somehow ended up with a different composition from the cloud of dust and gas that preceded it.”

The results, which were recently published in Nature, come at an interesting time for studies on the composition of the Sun. Between 2001 and 2004 a NASA space probe called Genesis has been collecting samples of solar wind. The probe crashed down to Earth in 2004 but its precious cargo of solar particles was retrieved. Those samples are now being sorted and catalogued and will soon be available for analysis.

“In many ways, our measurements on the lunar samples were our first attempt at scoping the experimental difficulties we might have in measuring the Genesis samples,” says Dr Ireland. “As it turns out, the analysis we have undertaken using the SHRIMP has proved to be very successful.

“The real question our finding raises is why this solar composition appears unrelated to the composition of the planets, the largest rocky bodies in the solar system, or to refractory inclusions from meteorites which have been regarded as solar condensates. Further study of samples from the Genesis mission may have the answers.”

More info: Trevor.Ireland@anu.edu.au

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